Method of treating hollow blooms in rolling mills

ABSTRACT

THE OPERATION OF DRIVERS FOR THE ROLLS OF A ROW OF ROLLS STANDS IN A ROLLING MILL FOR TREATMENT OF HOLLOW BLOOMS WHICH ARE MOUNTED ON MANDRELS AND ARE CONVERTED INTO TUBULAR ARTICLES DURING A PASS THROUGH THE MILL IS REGULATED IN SUCH A WAY THAT THE SPEED OF THOSE ROLLS WHICH ENGAGE THE BLOOM IS REDUCED WHEN THE LEADING END OF THE BLOOM REACHES THE ROLLS OF THE SECOND STAND AND, WHERE APPROPRIATE, AGAIN WHEN THE LEADING END OF THE BLOOM REACHES THE ROLLS OF ONE OR MORE NEXT-FOLLOWING STANDS, AND WHEN THE TRAILING END OF THE BLOOM LEAVES THE ROLLS OF THE FIRST STAND AND, WHERE APPROPRIATE, AGAIN WHEN THE TRAILING END OF THE BLOOM LEAVES THE ROLLS OF ONE OR MORE NEXT-FOLLOWING STANDS. THE REDUCTION IN THE SPEED ROLLS IS A FRACTION OF THE DIFFERENCE BETWEEN THE SPEED OF THE MANDREL PRIOR AND SUBSEQUENT TO ENTRY OF THE LEADING END OF THE BLOOM INTO A PARTICULAR STAND OR A FRACTION OF THE DIFFERENCE BETWEEN THE SPEED OF THE MANDREL PRIOR AND SUBSEQUENT TO ISSUE OF THE TRAILING END OF THE BLOOM OUT OF A PARTICULAR STAND.

Jan. 5, 1971 7 G. PFEIFFER ETAL 3,552,170

METHOD OF TREATING HOLLOW BLOOMS IN ROLLING MILLS Filed Feb. :5, 1969 R, I R8 c ,0 00 0 0000'00005 3 a, a, a, 0,5, a a, 5

R1- R8 I at 00000000 1 GOQQQDOQXF F1 4 CDQOOOOOC) QQUUUGLKJ 61 G2 G3 64 G5 G7 6 /n ven furs Gerd Pf e/ f fer H Bil/er a? Schrey hlt/t'l l (I f Q L S he/r Attorney United States Patent US. Cl. 72-209 10 Claims ABSTRACT OF THE DISCLOSURE The operation of drives for the rolls of a row of roll stands in a rolling mill for treatment of hollow blooms which are mounted on mandrels and are converted into tubular articles during a pass through the mill is regulated in such a way that the speed of those rolls which engage the bloom is reduced when the leading end of the bloom reaches the rolls of the second stand and, where appropriate, again when the leading end of the bloom reaches the rolls of one or more next-following stands, and when the trailing end of the bloom leaves the rolls of the first stand and, where appropriate, again when the trailing end of the bloom leaves the rolls of one or more next-following stands. The reduction in the speed rolls is a fraction of the 'dilference between the 7 speed of the mandrel prior and subsequent to entry of the leading end of the bloom into a particular stand or a fraction of the difference between the speed of the mandrel prior and subsequent to issue of the trailing end of the bloom out of a particular stand.

BACKGROUND OF THE INVENTION The present invention relates to a method of treating hollow blooms in continuous rolling mills. More particularly, the invention relates to improvements in a method of treating hollow blooms which are mounted on mandrels during travel between the driven rolls of successive roll stands in a rolling mill. Still more particularly, the invention relates to improvements in a method of converting thick-walled hollow blooms, especially those which are produced in roll piercing mills, into elongated tubular intermediate products or tubes of reduced wall thickness and increased length. Such intermediate products can be treated in a further rolling mill to yield tubes of desired wall thickness.

A continuous rolling mill normally comprises a row of 2-high stands whose rolls are driven independently of rolls in the other stands and wherein the blooms are treated during a single pass. The distance between the stands is such that the bloom is treated simultaneously by the rolls of two or more stands during a certain stage of the pass.

In presently known continuous rolling mills for treatment of tubular blooms which are mounted on mandrels, the rolls of successive stands are driven at increasing speeds to compensate for elongation of that part of the bloom which extends forwardly beyond the first or foremost stand. A drawback of such conventional rolling mills is that the ends and often also certain median parts, of intermediate products are formed with so-called bulges, i.e., with portions of greater diameter and greater wall thickness. Such bulges cause excessive stressing of the rolling mill and/ or of machines which subject the intermediate product to one or more additional treatments. Furthermore, those parts of finished tubes which are obtained upon further deformation of bulges are often de- 3,552,170 Patented Jan. 5, 1971 ice,

fective and must be discarded. Severing of portions of finished tubes produces much waste and consumes too much time.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of treating hollow blooms in continuous rolling mills in such a way that the formation of bulges is prevented or that the bulges are much less pronounced than in rolling mills which are operated in accordance with presently known methods.

Another object of the invention is to provide a method of treating hollow blooms in rolling mills in such a way that the percentage of waste is reduced and that the time required for secondary treatment is reduced to a minimum.

A further object of the invention is to provide a method of treating hollow blooms in rolling mills in such a way that the likelihood of damage to parts of the rolling mills is much less pronounced than in rolling mills which are operated in accordance with presently known methods.

Still another object of the invention is to provide a rolling mill for treatment of hollow blooms in accordance with the above-outlined method.

The method of our invention is employed to regulate the treatment of hollow blooms in continuous rolling mills wherein the bloom is mounted on a mandrel and is conveyed through a row of several (preferably five or more) roll stands. The rolls of each stand are driven independently from the rolls of the other stands and the peripheral speed of rolls in successive stands exceeds the speed of rolls in preceding stands by a ratio which is always dependent on elongation of the bloom. The method comprises the steps of conveying a bloom lengthwise so that its leading and trailing ends move through and beyond successive stands whereby the speed of the mandrel increases whenever the leading end reaches the rolls of a stand and whenever the trailing end moves beyond the rolls of a stand, and reducing the rotational speed of those rolls which engage the bloom by a fraction of the increase in speed of the mandrel when the leading end reaches the region of the rolls of the second stand, and when the trailing end leaves the rolls of the first stand. If necessary, the speed of those rolls which engage the bloom is reduced again when the leading end of the bloom reaches the region of rolls of the third, fourth, etc. stand and when the trailing end leaves the rolls of the second, third, etc. stand.

Rotational speed of those rolls which engage the bloom is preferably reduced by the factor a a-l-b Ad wherein a is the coefiicient of friction between the mandrel and the bloom, b is the coefiicient of friction between the rolls and the bloom, and A d is the difference (a) between the speeds of the mandrel prior and subsequent to the entry of the leading end of the bloom into the second stand and, if necessary, into the third etc. stand or between the speeds of the mandrel prior and subsequent to the issue of the trailing end of the bloom out of the first stand and if necessary, the second, third etc. stands. The value a can be replaced by a factor which is proportional to the difference between the interval required by the leading end of the bloom to advance from the foremost stand to the second stand and the interval required by the trailing end to cover the same distance, i.e., to move from the foremost stand to the second stand. Such replacement is made aft-er the leading end or the trailing end advances from the second stand into the region of rolls of the third stand. The length of aforementioned intervals is preferably measured by electrical or electronic timer means, and the speed or rolls which engage the bloom is changed automatically by electronic means in accordance with the result of such measurement, i.e., as a function of the difference between the two intervals.

The bloom normally consists of steel or other metallic material, and the speed of rolls which engage the bloom is in our method reduced by a certain amount. The reduction in speed can be repeated when the leading end of the bloom reaches the rolls in the third, fourth, etc. stand and/or when the trailing end of the bloom leaves the rolls in the second, third, fourth, etc. stand. In many instances, it suffices if the speed of those rolls which engage the hollow bloom is reduced when the leading end moves into engagement with the rolls of the second, third and eventually fourth stand and/or when the trailing end moves beyond the rolls in the first, second, third and eventually fourth stand.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The operation of the rolling mills itself, however, together with additional features and advantages of our method, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic side elevational view of a continuous rolling mill with six stretching stands and two non-stretching stands, a bloom being shown prior to entry of its leading end into the foremost stand;

FIG. 2 illustrates the rolls of the rolling mills and the bloom in a position when its leading end reaches the rolls of the second stand;

FIG. 3 is a similar view but showing the bloom in engagement with all of the rolls;

FIG. 4 shows the bloom in a position when its trailing end leaves the foremost stand; and

FIG. 5 shows the trailing end of the bloom when it leaves the rolls of the second stand.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, there is shown a rolling mill with six stretching roll stands G1-G6 and two nonstretching roll stands G7, G8 each of which comprises a set of driven rolls flanking the path for a tubular metallic bloom 10 which surrounds a mandrel 11. The rolls R1 of the first stand G1 are driven by a motor or analogous prime mover M1. This motor is of the variablespeed type or it drives the rolls R1 by way of a variablespeed transmission T1. The motor M2 drives the rolls R2 of the second stand G2 by way of a variable-speed transmission T2. Similar motors are transmissions are employed to drive the rolls R3-R8 of the stands G3- G8. Each set of rolls of a stand can be driven independently of the sets of the rolls in the other stand or stasds.

FIG. 1 illustrates that stage of operation when the leading end of the hollow blooms 10 approaches the rolls R1 in the first stand G1. In FIG. 2, the leading end of the bloom 10 is about to be engaged by the rolls R2. In FIG. 3, the bloom 10 is engaged by each pair of rolls (R1R8). In FIG. 4, the trailing end of the bloom 10 is about to leave the rolls R1. FIG. 5 illustrates that stage of treatment in which the trailing end of the bloom 10 is about to leave the rolls R2. When the entire bloom 10 moves beyond the rearmost rolls R8, it is converted into an intermediate product whose external diameter is constant from end to end and is a fraction of the external diameter of the bloom 10. The length of the bloom 10 increases progressively as the bloom moves forwardly toward and past the rolls R6 of the rearmost stretching stand G6. In the non-stretching stand G7 and G8 the rolls R7 and R8 of which have always the same peripheral speed as the rolls R6 the bloom is smoothed.

In accordance with the method of our invention, the formation of one or more bulges on the bloom 10 is prevented by regulating the speed of rolls R1-R8 during travel of the leading end of the bloom 10- from the rolls R1 toward the rolls R8 and/or by regulating the speed of the rolls R2-R8 during travel of the trailing end of the bloom from the rolls R1 toward the rolls R8. As stated before, the peripheral speed of rolls R2 exceeds the peripheral speed of rolls R1, the peripheral speed of rolls R3 exceeds the peripheral speed of rolls R2, and so forth, in order to account for elongation of the bloom during travel through the gaps between successive pairs of rolls. When the leading end of the bloom 10 reaches the rolls R2, the speed of the rolls R1 (namely, those rolls which already engage the bloom 10 prior to the entry into the following rolls), is reduced to a predetermined extent. Also, when the leading end of the bloom 10 advances toward and is engaged by the rolls R3, the speed of rolls R1, R2 is reduced again. This reduces the likelihood of bulge formation. The reduction in the speed of those rolls which engage the bloom 10 is a fraction of the difference between the speeds prior and subsequent to the entry of the leading end of the bloom 10 into the second, third, etc. stretching stands G2-G6.

In the same way, when the trailing end of the bloom 10 leaves the foremost rolls R1, the speed of rolls R2-R8 (namely, of those rolls which still engage the bloom) is reduced by a fraction of the difference between the speeds of mandrel 11 prior and subsequent to the issue of the trailing end of the bloom 10 out of the rolls R1. Analogously, the speed of rolls R3-R8 can be reduced when the trailing end of the bloom 10 leaves the rolls R2, R3, R4, R5.

The reduction in the speed of those rolls which engage the bloom whenever the leading end of the bloom 10 reaches the rolls R2, R3, etc. is preferably selected in the following way: It equals the difference between the speed of the mandrel 11 prior and subsequent to entry of the leading end into a particular stand G multiplied by the coefficient of friction between the mandrel 1'1 and bloom 10 and divided by the sum of coefficient of friction between the mandrel and bloom on the one hand and the bloom and the respective rolls on the other hand. Thus, the reduction (Av) in the speed of rolls is a function of the difference (Ad) in speeds of the mandrel 11 and a function of the factor wherein a is the coefiicient of friction between the bloom 10 and mandrel 11, and b is the coefficient of friction between the bloom 10 and a particular roll R. As stated above,

a AUm'Ad With regard to the trailing end of the bloom 10 the factor Ad equals the difference between the speed of mandrel prior and subsequent to the issue of the trailing end of the bloom 10 out of the rolls R1, R2, etc.

EXAMPLE It is assumed that the rolling mill comprises six stretching stands G1-G6) and two non-stretching stands (G7, G8) and that the speed of r0llsR1-R8 is regulated only during that phase of a pass during which the trailing end of the bloom 10 moves from the rolls R1 toward the rolls R6. The distance between the gap defined by the rolls R1 and the gap defined by rolls R6 is 7 meters, the initial length of the bloom 10 is 5 meters, and the length of the intermediate product is 20 meters. The initial peripheral speeds of rolls R1 to R6 are as follows:

Meters per second The initial speed of the mandrel 11 is 3 meters per second. This speed increases by Ad =0.25 meter per second (to 3.25 meters per second) after the trailing end of the bloom leaves the rolls R1 of the foremost stand G1. The speed of the mandrel '11 increases by Ad =0.80 meter per second (to 3.80 meters per second) when the trailing end of the bloom 10 leaves the rolls R2 of the second stand G2. The speed of the mandrel 111 rises by Ad '=1.20 meters per second (to 4.20 meters per second) when the trailing end of the bloom 10 leaves the rolls R3 of the third stand G3.

In accordance with the presently prevailing practice, the aforementioned speeds of the rolls R1-R8 remain unchanged while the leading end or the trailing end of the bloom advances from the rolls -R1 toward and beyond the rolls R6. In accordance with the method of our invention, the speed of rolls R2R8 is regulated in the following way while the trailing end of the bloom 10 moves from the rolls R1 toward and beyond the rolls R2, R3 R8:

If a (coefficient of friction between the mandrel 11 and bloom 10) equals 0.09 and b (coefficient of friction between the bloom and the rolls) equals 0.25, then and the speed of rolls R2-R8 is reduced by meter per second. When the trailing end of the bloom 10 moves beyond the rolls R2, the speed of rolls R3-R8 is reduced by Av =0.80-0.265=0.21 meter per second. When the trailing end moves beyond the rolls R3, the speed of rolls R4'R-8 is reduced by Av =1.20-.265=0.32 meter per second etc. The following table shows the speeds of rolls R1R8 during travel of the trailing end of the bloom from the rolls R1 toward and beyond the rolls R2 and R3:

Speed of rolls when the trailing end of bloom 10 leaves the rolls Stand Rolls R1, m./s. R2, m./s. R3,m./s;

As a rule, it sufiices to consider the changes in speed of mandrel -11 during movement of the trailing end of the bloom beyond the foremost stand or stands, for example, beyond the stands G1, G2, G3.

The same method can be employed during movement of the leading end of the blank 10 into engagement with the rolls R2 R8. Thus, the speed of the foremost rolls R1 is reduced by the value a -Ad I when the leading end reaches the rolls R2. The speed of rolls R1, R2, is reduced when the leading end reaches the rolls R3, and so on.

As a rule, the coefficient b (friction between the blooms) and the rolls varies little or not at all. However, the

coefficient a (friction between the mandrel and the blooms) varies considerably in response to wear on the mandrel as well as in dependency on the condition and type of the material of the bloom. The method of constantly determining the value of a or an equivalent factor constitutes another feature of our invention. This involves measuring the length of the interval required by the leading end of the bloom to move from the rolls R1 to the rolls R2, measuring the length of the interval required by the trailing end of the bloom to cover the distance between the rolls R1 and R2, and determining the difference between the lengths of these intervals. It was found that such difference is a good indicator of the coefficient of friction between the mandrel and the bloom. For example, if the leading end requires an interval of 600 milliseconds and the trailing end requires an interval of 530 milliseconds to move from the rolls R1 to the rolls R2, the difference of 70 milliseconds is indicative of a particular coefficient a in a particular continuous rolling mill and when the bloom consists of a particular type of steel. For example, if the difference of 70 milliseconds corresponds to a coefficient a=0.09, the difference of 90 milliseconds can indicate a coefficient a=0.l1. Thus, the difference in speeds of leading and trailing ends of a bloom between the first and second roll stands can be used as a parameter for calculation of the coefficient of friction between the bloom and the mandrel.

If a mandrel is used repeatedly, i.e., if the initial coefiicient a is known, the speed of rolls in the stands of the rolling mills can be varied in accordance with a program which is established by consideration of the fact that the coefiicient a varies in response to repeated utilization of the mandrel. The speed of rolls can be varied automatically by electronic means in the course of a rolling operation. For example, an electronic timer mechanism (not shown) is started automatically by the rolls R1 when the leading end of the bloom enters the first or foremost roll stand, and the mechanism is automatically arrested when the leading end enters the second stand. The mechanism is started again by rolls R1 when the trailing end of the bloom moves beyond the foremost stand and is arrested when the trailing end leaves the second stand. Thestarting and stoppage of the mechanism can be caused by change of stresses upon the rolls of the stands G1, G2 when the leading or trailing end of the bloom enters or leaves such stands. The electronically determined difference between the two measured intervals is utilized for automatic regulation of speeds of rolls in the remaining stands by effecting appropriate changes in speeds which are listed in the preceding table. It is normally sufficient to carry out corrections only when the difference between the two measurements exceeds a predetermined minimum value. For example, the minimum value is 70 milliseconds if the bloom with dimensions 116 x 3.25 mm. consists of low carbon steel, if the distance between the stands is 950 mm., if the mandrel consists of 28 NiCrMoVlO and has a ,u=0.09, if each roll is a nodular iron with a ,u.=0.25, and if the ratio of elongation )\=l24.4.

It was further found that it is not always necessary to reduce the speed of certain rolls whenever the leading or trailing end of the bloom respectively moves into or beyond a roll stand. For example, when the rolling mill comprises five roll stands, it is sufficient to reduce the speed of rolls R2R4 when the trailing end leaves the rolls R1 and to thereupon drive the rolls R2R4 at an unchanging speed. Repeated reduction in the speed of rolls is normally desirable in a large rolling mill with six or more roll stands. If the mill comprises eight stands, as shown in the drawings, it is normally suflicient to reduce the speed of rolls R2R8, R3-R8 and R4R8 when the trailing end of a bloom respectively leaves the rolls R1, R2 and R3. Analogously, the speed of rolls R1, R1

The method of our invention has been conceived on the basis of the following considerations:

During the first stage of a pass (FIGS. 1 and 2), i.e., when the leading end of the bloom enters successive stand G1 to G8, the speed of the mandrel 11 increases as a function of the peripheal speed of rolls in successive stands. The speed of the mandrel 11 assumes a value corresponding to an average speed of rolls which engage the bloom provided that the pressure which the rolls of successive stands transmit to the bloom is the same. If the pressure transmitted by a particular pair of rolls is higher, the speed of the mandrel closely approximates the speed of such particular rolls.

During the second stage (FIG 3), all of the rolls R1- R8 engage the bloom 10 and the speed of the mandrel 11 is constant.

During the thid and final stage (FIGS. 4-5), the mandrel 11 is accelerated whenever the trailing end of the bloom 10 moves beyond a pair of rolls. The speed of the mandrel approximates the average speed of rolls which remain in engagement with the bloom.

It was found that, the speed of the mandrel 11 changes stepwise (abruptly) whenever the leading end of the bloom enters or the trailing end of the bloom leaves a set of rolls. These sudden changes in speed of the mandrel influence the speed of the bloom in such a way that each increase in speed of the mandrel causes an increase in speed of the bloom and attendant formation of a bulge which is due to an discontinuity in flow of material of the bloom. The aforementioned regulation of the speed of rolls during the first and/or last stage of a pass insures that the above mentioned discontinuity is prevented and that the intermediate product is free of bulges or at least nearly free of bulges with respect to intermediate products produced by conventional rolling methods.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. A method of regulating the treatment of hollow blooms in continuous rolling mills wherein a bloom is mounted on a mandrel and is conveyed through a row of at least three roll stands, the rolls of each stand are driven independently of the rolls of the other stands and wherein the peripheral speed of rolls in successive stands exceeds the speed of rolls in preceding stands by a ratio which is always dependent on elongation of the bloom, comprising the steps of conveying a bloom lengthwise so that its leading and trailing ends move through and beyond successive stands, the speed of the mandrel increasing whenever the leading end reaches the rolls of a stand and whenever the trailing end moves beyond the rolls of a stand; and reducing the rotational speed of those rolls which still engage the bloom by a fraction of the increase in speed of the mandrel when the trailing end leaves the rolls in the first stand.

2. A method of regulating the treatment of hollow blooms in continuous rolling mills wherein a bloom is mounted on a mandrel and is conveyed through a row of at least three roll stands, the rolls of each stand are driven independently of the rolls of the other stands and wherein the peripheral speed of rolls in successive stands exceeds the speed of rolls in preceding stands by a ratio which is always dependent on elongation of the bloom, comprising the steps of conveying a bloom lengthwise so that its leading and trailing ends move through and beyond successive stands, the speed of the mandrel increasing whenever the leading end reaches the rolls of a stand and whenever the trailing end moves beyond the rolls of a stand; and reducing the rotational speed of those rolls which already engage the bloom by a fraction of the increase in speed of the mandrel when the leading end enters the rolls in the second stand.

3. A method as defined in claim 1, and reducing the rotational speed of the rolls which still engage the bloom when the trailing end leaves the rolls of the second stand.

4. A method as defined in claim 2, and reducing the rotational speed of the rolls in the first and second stands when the leading end reaches the rolls of the third stand.

5. A method as defined in claim 1, further comprising the additional step of reducing the speed of those rolls which engage still the bloom when the trailing end of the bloom leaves the rolls in the third stand.

6. A method as defined in claim 2, and reducing the speed of rolls in the first and second and third stands when the leading end of the bloom reaches the roll of the fourth stand.

7. A method as defined in claim 1, wherein the rotational speed of those rolls which still engage the bloom is reduced by a a-l-If wherein a is the coefiicient of friction between the mandrel and the bloom, b is the coefficient of friction between the bloom and the rolls, and Ad is the difference between the speed of the mandrel prior and subsequent to the issue of the trailing end of the bloom out of the first stand and, if necessary, the second, third, etc. tands.

8. A method as defined in claim 2, wherein the rotational speed of those rolls which already engage the bloom is reduced by a a -Ad wherein a is the coeflicient of friction between the mandrel and the bloom, b is the coefficient of friction between the rolls and the bloom, and Ad is the difference between the speed of the mandrel prior and subsequent to the entry of the leading end of the bloom into the second stand and, if necessary, into the third, etc. stands.

9. A method as defined in claim 1, further comprising the step of reducing the speed of those rolls which still engage the bloom when the trailing end of the bloom leaves the second stand by wherein a is a factor which is proportional to the difi'erence between the interval required by the leading end to advance between the two foremost stands and the interval required by the trailing end to advance between the two foremost stands.

10. A method as defined in claim 9, further comprising the steps of measuring the length of said intervals and changing the speed of those rolls which engage the bloom in dependency on the result of such measurement.

References Cited UNITED STATES PATENTS 2,044,358 6/1936 Korbuly 72209 3,392,565 7/1968 Rodder 72-209X MILTON S. MEHR, Primary Examiner 

